Pneumatic tires are assembled on rims, inflated with air, and mounted on a vehicle. When a vehicle is traveling, it is this internal air pressure that bears the load. However upon puncture or the like, air escapes from the pneumatic tire, rendering the tire unsuitable to bear the load. Specifically, the load supported by the air pressure becomes supported by the sidewall portions, causing the sidewall portions to greatly deform. As a result, travel is impeded.
Pneumatic tires are known that take into account such issues. Such tires are capable of run-flat travel (traveling in a deflated state due to a puncture or the like) and are provided with a reinforcing rubber layer to the inside of the sidewall portions which increases the flexural rigidity of the sidewall portions. In other words, deformation of the sidewall portions of such pneumatic tires is suppressed, allowing the pneumatic tire to travel even in a state in which some of the air inflating the pneumatic tire has escaped and a large load is borne by the sidewall portions.
Conventional tires, such as the pneumatic tire described in Japanese Unexamined Patent Application Publication No. 2000-289409, have an object of improving the performance of run-flat travel by improving a sectional shape of the tire to obtain an optimal sectional shape by which maximum stress applied to the tire can be reduced while minimizing increase in tire weight. This pneumatic tire satisfies the following expressions:Ra/D≤0.08;Rb/D≤0.08;0<φa≤50 deg.; and0<φb≤50 deg.;
where, in a tire meridional section in a standard state in which the tire is mounted on a standard wheel rim, inflated to a standard inner pressure, and not loaded, Ra is a radius of curvature of a first arc having a center Oa on a tire axial direction line X passing through a midpoint between a first point A and a second point B and on a tire inner cavity side and in contact with a tire center line at the first point A, the first point A being a point where a tire radial direction line Y passing through a wheel rim width position of the standard wheel rim intersects the tire center line passing through a midpoint of a thickness of the tire on a tread portion side and a second point B being a point where the tire radial direction line Y intersects the tire center line on a bead portion side, Rb is a radius of curvature of a second arc having a center Ob on the tire axial direction line X and on the tire inner cavity side and in contact with the tire center line at the second point B, φa is an angle formed by the tire axial direction line X and a straight line Oa-A connecting the center Oa and the first point A, φb is an angle formed by the tire axial direction line X and a straight line Ob-B connecting the center Ob and the second point B, and D is an outer diameter of the tire in the standard state.
However, though the steering stability performance of pneumatic tires capable of run-flat travel can be increased by increasing the thickness of the reinforcing rubber layer in the tire width direction to increase the vertical stiffness in the tire radial direction, such an increase in the thickness of the reinforcing rubber layer leads to an increase in the volume of the reinforcing rubber layer, which tends to cause an increase in rolling resistance. On the other hand, reducing the thickness of the reinforcing rubber in the tire width direction to reduce rolling resistance tends to cause a decrease in durability performance when run-flat traveling (run-flat durability performance).